1. Field of Invention
This invention relates generally to electronic compasses and more particularly to a system and method for calibrating a magnetic compass.
2. Description of Related Art
Electronic compasses provide compassing capabilities in a variety of applications such as automobiles, airplanes, boats, and personal handheld devices. Magnetic compasses are one type of electronic compass that provide a means for determining a compass heading. As examples, the compass heading may indicate North or South. Other examples of compass headings are also possible.
A magnetic compass may include two or more magnetometers. A magnetometer is a device capable of sensing a magnetic field or a component of a magnetic field in one or more directions. A magnetic compass may use magnetometers to sense a magnetic field in proximity to the magnetic compass and to determine a compass heading.
A magnetic compass may be calibrated in order to ensure the compass provides an accurate compass heading. One form of compass calibration occurs by (i) using external calibration equipment to apply a magnetic field in close proximity to the compass, and (ii) trimming compass signals produced by the compass in response to how the magnetic field affects the compass signals.
Compass signals are signals produced by a compass and may include, but are not limited to, output signals of the magnetometers. One way to trim compass signals is to adjust the compass components that produce the compass signal. Trimming compass signals may be used to compensate for variation in the compass components that produce the compass signals.
Various entities may calibrate a compass by trimming compass signals. For example, an entity such as a compass manufacturer may trim compass signals during the process of manufacturing a compass, prior to distributing a manufactured compass to its customers. As another example, an entity such as a customer that receives a manufactured compass from a compass manufacturer may trim compass signals prior to integrating the compass with a given device, such as a wireless phone, or after integrating the compass with the given device.
Calibrating a magnetic compass by trimming compass signals could involve applying a magnetic field at the magnetic compass in order to produce a change in the magnetic field acting on the magnetic compass. Applying a magnetic field at the magnetic compass has typically consisted of using a device external to the magnetic compass to create the magnetic field. Devices for generating magnetic fields at magnetic sensors are well known. For example, U.S. Pat. No. 6,661,223 provides that a magnetic field can be generated by several known techniques such as using a Helmholz coil or using a coil wrapped around a ferromagnetic material such as iron.
The need to use calibration equipment external to a magnetic compass during the calibration of a magnetic compass may be a burden to the person/entity calibrating the magnetic compass because of the expense, set up, maintenance, and use, of the calibration equipment. It would thus be useful if a magnetic compass could be calibrated by trimming compass signals without the need to use calibration equipment external to the magnetic compass during the calibration process.